This invention pertains to rotating turbomachinery and, more particularly, to a device for containing rotor blades by absorbing the rotational energy thereof when inadvertently contacted by the blades.
Gas turbine engine turbomachinery typically operates at high rotational speeds and thus possesses high kinetic energy, even when the rotating structures are fabricated out of modern lightweight alloys or composites. At times, entire rotor blades or parts thereof have been inadvertently shed from rotors due to the impact of foreign objects entrained in the propulsive gas stream or from any of a number of other well-known causes. Since the shed blades possess such a high kinetic energy, means should be provided to contain them within the turbomachinery to minimize further secondary damage. Hence, there is usually provided a stationary, rigid containment ring which circumscribes the rotating blade tips in radial spacial relationship. Typically, this ring is very heavy and is constructed of a high-strength metal. However, as the released blade contacts this containment ring, it is impacted, in turn, by the following blades which are still attached to the rotor and violent forces are transmitted to the ring, the blades and their associated support structure. In particular, large circumferential torque loads are transmitted to the internal engine frame and large bending moments are created in the rotor blades. In the event that these members become over-stressed, the possibility of secondary damage exists even though, strictly speaking, the released blade has been contained. It is therefore desirable to provide a means for absorbing in a more gradual manner the energy from a released turbomachinery blade, thereby limiting secondary damage and maintaining the various forces and torques at a lower level.
Closely akin to the problem of inadvertently released blades is the situation where a temporary imbalance of, or transient load upon, the turbomachinery rotor temporarily reduces the concentricity between the rotor blades and the circumscribing containment ring. Since turbomachinery efficiency is an inverse function of clearance, the clearance between the blade tips and the containment ring is very tight so that small variations in concentricity can cause a blade to rub against the ring. When the rub is severe, large bending moments can be created in the blade. If these stresses are large enough, blade failure can occur and the aforementioned secondary damage results. It is therefore desirable to provide a more forgiving containment structure which will gradually absorb the rotational energy of a rubbing rotor rather than contribute toward the production of large stresses in the rotor blades.